Weldability

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Weldability
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Weldability

• Capacity to be welded to specific structure that
  has certain properties and characteristics
• Factors affecting weldability
• Alloying elements
• Impurities
• Inclusions
• Grain structure
• Processing history
• We will look at
• Aluminum
• Alloy steel
• Stainless steel
• Copper
• Plain carbon steel

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• We should have strong knowledge of the metallurgy
  of the metal, solidification, micro-structural
  changes, strength etc.
• Preparation of the workpiece selection of filler
  metal etc.

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Process selection

• Type of metal or alloy to be joined
• Thickness of the joint
• Cost of the process
• Skill requirement
• welding position and location
• Appearance
• Mass welding (Automation)
• Medium quantity (manual /Automation)
• few pieces (usually manual)

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A few general rules

• 1. Low/medium carbon and plain carbon steel
• Usually convert to ferrite and cementite during
  normal cooling rate during welding
• 2. Steel which always convert to martensite (0.3-
  0.4 steel with 1-2 Mn and up to 1 Cr)
• Usually pre-heat to 400 F helps in getting some
  ferrite and cementite conversion also
• High alloy steel (Cr-Ni, Cr-Ni-Mo)
• Cooling rate has no or little effect on the weld
• Cracking can usually be attributed to the
  chemical properties of the electrode

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Problems faced for welding stainless steel

• Creation of brittle sigma phase at the Grain
  boundary can reduce the strength drastically
• Happens when the Cr/Ni Steel is heated to between
  950 and 1850 F.
• Periodic annealing at 1800 helps chromium etc to
  diffuse back in to normal state
• chromium carbide precipitation
• When austenitic steel is heated to 1000-1300 F
  carbon combines chromium and precipitates at the
  grain boundary.
• However steel with titanium additive helps in
  avoiding this problem

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Stainless steel

• Three general rules
• Very high thermal coefficient of thermal
  expansion use fixtures, jigs etc
• Because of this stainless steel is hard to weld
  and care has to be taken
• Low thermal conductivity Energy input can be low
• High electric impedance smaller electrodes for
  resistance welding

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• Low carbon steel
• Can be welded without difficulty
• Alloy steel with Cr/Mo etc
• There is a tendency for weld to crack. Entire
  weld and most HAZ will transform into martensite
  (unless post heated to 800 F)
• Avoid restrained structure to avoid residual
  stresses
• Use low hydrogen electrodes
• Submerged arc (GMA, TMA etc)

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Some other Common alloys

• Aluminum
• Weldable with very high heat input rate because
  of thermal conductivity
• Problem of welding Al with steel or copper form
  brittle inter-metallic compounds at the weld
• Can be avoided by coating steel/copper with
  Aluminum/silver/tin
• Using ultra-sonic welding
• Copper Similar to Al
• Magnesium
• Fast oxidation rate
• Weldable with protective shield of inert gas
• Ni Alloys Similar to stainless steel

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Weldability
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Effect of increased temperature

• Helps remove oxides to a certain extent
• Helps grain growth and recrystallization
• Creates ductile joint
• Good weld with less deformation
• Effects diffusion reduces voids at the joint

Schematic illustration of recovery,
recrystallization and grain growth on mechanical
properties of weld
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• Mid Term Test covers topic until this slide

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• High temperature can create good weld with less
  deformation
• Close to melt point 10 deformation
• Effects surface hardness
• Surface oxide layers, oil film is the biggest
  problem
• Oxide layer is brittle
• Lateral movement helps (ultrasound welding

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• Metallic bridges due to pressure
• Elastic deformation
• Non elastic contact
• Now when the pressure is removed the parts can be
  separated only with a force which can counter
  elastic force in the welded region
• Thus softer material has better adhesion

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• Examples
• Forge welding
• Cold welding
• Friction welding
• Diffusion welding

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• Forge weld
• High temp. / high press.
• Very old process (1000 bc)
• Parts are heated and hammered together
• Cold welding
• Pressure through dies and rolls
• Best for two similar metals
• For mutually soluble metals causes brittle joint

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Joining Plastics

• Fusion weld, ultrasonic, friction, adhesion and
  mechanical means
• Thermosetting plastics can be welded by heating
  the joint by hot air
• Polyvinyl chloride, Polyethylene
• Adhesion using bonding chemicals
• Thermoset plastics are usually fastened or bolted
  because they can be softened

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Solidification of weld metal

• When no filler metal is used, the workpiece melts
  and re-solidifies
• Solidification process occurs just like in
  casting
• Columnar grains are created
• Usually cooling in ambient temperature create
  coarse grains so they usually have low strength
• However the strength can be controlled by weld
  metal selection

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• Heat treatment
• preheating for controlling the cooling rate
• A specially for aluminum and copper because of
  high thermal conductivity
• HAZ
• Properties and microstructures off HAZ depends
• Rate of heat input
• temperature to which the zone is raised
• Thermal conductivity

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Welded joints

• Weld metals have three zones of temperature
• Figure
• Metallurgy and property depend on
• the zones.
• filler metal used
• Welding process

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Weld deposit

• 1/3 austenite 2/3 ferrite
• Geometrical pattern of the ferrite within the
  Grain boundary is called Widmannstatten
  structure
• Consists of white interlaced masses distributed
  throughout the grain (ferrites)
• Strength of this course structure can be as low
  as 1/5 of the body

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Grain refiners

• Usually added to the filler Metal. Basically they
  provide nuclei such as oxides and nitrides (
  heterogeneous nucleation)
• Grain refinement can also provide finer grain
  with finer distribution of impurity such as FeS
  and improved ductility
• Fusion zone
• Columnar grain at one end and perlite and ferrite
  at the other end

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Multiple pass low carbon weld

• Multiple pass can use the heat from second pass
  to refine grain from the first pass
• Cool first bead to below 727 ºC (above 540 ºC)
  then apply next layer
• Properties improve especially strength and
  toughness
• If the first layer is at room temperature or
  above 727 ºC the property improvement is very
  small

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Weld pool solidification

• Nucleation of the solid liquid interface
• Columnar growth away from heat loss
• Also can have equiaxed growth or planar growth
  similar to casting
• Temperature gradient G
• Rate of advance of solid interface R the
• G/R is very high we can get Planar growth

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Comparison of various steels with respect to
thermal expansion etc.
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End of Class